1. Field of the Invention
The present invention relates in general to an integrated semiconductor device including a DC-DC converter control circuit of a pulse width modulated (PWM) type which can operate at a low voltage and which outputs an output voltage used as a self-bias voltage as well, and more particularly to a switching regulator having a function for automatically changing a boosting operation and a deboosting operation over to each other.
2. Description of the Related Art
FIG. 3 shows a circuit diagram of a conventional switching regulator having a function for automatically changing a boosting operation and a deboosting operation over to each other.
The boosting/deboosting automatic changing function is a function for automatically changing a boosting operation and a deboosting operation over to each other based on a value of an input voltage to obtain a desired output voltage.
Hereinafter, this circuit will be described with reference to FIG. 3. A DC-DC converter control circuit includes a bleeder resistor circuit 206, a reference voltage circuit 201, an error amplifier 204, a circuit 205 for shifting an output voltage of the error amplifier 204, a triangular wave oscillator 202, PWM comparator circuits 207 and 208, an activation oscillator 203, change-over switches 209 and 210, and output buffer circuits 211 and 212.
The bleeder resistor circuit 206 outputs a voltage which is obtained by dividing an output voltage VOUT of the switching regulator by a resistance ratio, The reference voltage circuit 201 is a reference voltage source for outputting a constant voltage of 1 V, for instance.
The error amplifier 204 outputs a voltage which is determined based on an electrical potential difference between the voltage of the reference voltage circuit 201 and the voltage obtained through the resistance ratio division with the bleeder resistors.
The error amplifier output voltage shifting circuit 205 outputs a voltage which is obtained by shifting an output voltage of the error amplifier 204. For this shift width, in general, there is adopted a method including shifting a voltage for an amplitude of a triangular wave signal of the triangular wave oscillator 202.
The PWM comparator circuit 207 compares the output of the triangular wave oscillator 202 with the output of the error amplifier 204 to thereby output a PWM control signal 221.
The PWM comparator circuit 208 compares the output of the triangular wave oscillator 202 with the output of the error amplifier output voltage shifting circuit 205 to thereby output a PWM control signal 222.
The activation oscillator 203 is an oscillator which can operate at a low power supply voltage of 1 V, for instance, and outputs an activation pulse 220.
The change-over switch 209 changes the PWM control signal 221 and the GND electric potential over to each other to output a driving signal 223 used to drive a P-channel MOSFET 213 through the output buffer circuit 211.
The change-over switch 210 changes the PWM control signal 222 and the activation pulse 220 over to each other to output a driving signal 224 used to drive an N-channel MOSFET 216 through the output buffer circuit 212.
The change-over switches 209 and 210 are circuits for, when the PWM control signals 221 and 222 have a voltage, equal to or larger than 2 V for example, enough for the switching regulator to normally operate, carrying out the change-over operation so as to output the PWM control signals 221 and 222, respectively.
When the PWM control signals 221 and 222 have a voltage not reaching a voltage enough for the switching regulator to normally operate, the circuits such as the error amplifier 204, the error amplifier output voltage shifting circuit 205, the triangular wave oscillator 202, and the PWM comparator circuits 207 and 208 can not normally operate. As a result, it is impossible to obtain the PWM signal which is to be essentially obtained. In such a case, the change-over switches 209 and 210 change the PWM control signals 221 and 222 over to the GND electric potential and the activation pulse 220 to output the GND electric potential and the activation pulse 220, respectively.
A boosting/deboosting automatic change-over switching regulator circuit includes the DC-DC converter control circuit, the P-channel MOSFET 213, a chopper coil 214, the N-channel MOSFET 216, Schottky barrier diodes 215 and 217, and a capacitor 218, and hence enables the boosting/deboosting automatic change-over operation.
When the low voltage driving is realized using the DC-DC converter control circuit having a plurality of channel outputs, it is effective that the circuit configuration as described above is adopted, and further that the output voltage VOUT of the switching regulator is used for a self-bias.
FIGS. 4A and 4B are timing charts explaining an operation sequence of the conventional DC-DC converter control circuit.
FIG. 4A shows a rise sequence of the DC-DC converter control circuit in the boosting operation. The boosting operation is an operation when a power supply voltage VDD of the switching regulator circuit is equal to or lower than a desired output voltage.
When the output voltage VOUT after the power supply voltage is applied is low, the change-over switches 209 and 210 are made to change sides over to activation sides to thereby output the GND electric potential and the activation pulse to a P-channel MOSFET drive terminal (hereinafter referred to as “a PDRV terminal” for short) 223 and an N-channel MOSFET drive terminal (hereinafter referred to as “a NDRV terminal” for short) 224, respectively. At this time, the switching regulator circuit carries out the boosting operation in accordance with the activation pulse to boost the output voltage VOUT. Thereafter, when the output voltage VOUT becomes a voltage at which the stable operation becomes possible, the change-over switches are made to carry out the change-over operation so as to output the PWM control signals, respectively, so that the operation becomes the normal PWM operation.
FIG. 4B shows a rise sequence of the DC-DC converter control circuit in the deboosting operation. The deboosting operation is an operation when the power supply voltage VDD of the switching regulator circuit is equal to or larger than the desired output voltage.
As in the boosting operation, when the output voltage VOUT after the power supply voltage is applied is low, the change-over switches 209 and 210 are made to change sides over to activation sides to thereby output the GND electric potential and the activation pulse to the PDRV terminal and the NDRV terminal, respectively. At this time, the switching regulator circuit carries out the boosting operation in accordance with the activation pulse to boost the output voltage VOUT. Thereafter, when the output voltage VOUT becomes a voltage at which the stable operation becomes possible, the change-over switches are made to carry out the change-over operation so as to output the PWM control signals, respectively, so that the operation becomes the normal PWM operation.
As described above, at the rise of the power supply voltage, the boosting output signal is changed from the activation pulse signal over to the PWM control signal as the output voltage VOUT increases from the low voltage to the voltage at which the stable operation can be carried out. Since the pulse signal corresponding to the output voltage VOUT is outputted using the PWM control signal, the output voltage VOUT becomes stable at a desired output voltage value (refer to JP 2002-233138 A (FIG. 11)).
However, the activation pulse signal is a pulse signal having a fixed waveform, and hence is not a pulse signal corresponding to the output voltage VOUT. Thus, the output voltage maybe unstable depending on the loads.
In case of an overload, it is conceivable that a time period required to carry out the boosting operation becomes longer, or no boosting operation is carried out. On the other hand, in case of a light load, it is conceivable that a time period required to carry out the boosting operation becomes shorter, and hence a ripple becomes large when the change-over operation is carried out so as to output the PWM control signals.